NOTE: This spoiler was sent in by Caboose.
The movie opens at City Hall in Capitol City, where the mayor is giving a speech about how crime in the city has skyrocketed. In front of his podium are several boxes filled with unsolved cases. The mayor is surrounded by many cops and K9 dogs; the only non-German shepherd K9 is Underdog (voiced by Jason Lee, but he doesn’t talk at this point of the movie). Right in the middle of the speech, Underdog starts barking and runs toward a certain unsolved case box; this causes panic and everyone is evacuated. The bomb squad checks out the box to find…..a ham. It becomes obvious that Underdog isn’t exactly the greatest K9 dog; actually, he’s the worst.
Feeling humiliated, Underdog drags his K9 suit off and goes for a walk. Cad (Patrick Warburton) sees the dog and takes him into a lab. After he gets put into a cage surrounded by other caged dogs, Underdog (now he starts talking) asks the dog next to him what this place is; all the dog says is that BAD stuff goes on in here. In the lab lobby, Cad watches the door with another security guard, Dan Unger (Jim Belushi). Dan stops a man trying to walk in saying the lab is closed for the night. The man shows his pass, and Cad identifies him as Dr. Simon Barsinister (Peter Dinklage I recognized him as Marlo from Nip/Tuck). Simon and Cad go into the lab, where they retrieve Underdog and plan to perform an experiment on him. Simon tells Underdog that it’s going to hurt a lot, which causes Underdog to sneeze into Simon’s mouth then jump out of his hands. As he runs around, Underdog damages a bunch of damage to the lab and knocks a bunch of chemicals onto himself. The chemicals change his body around and make him able to run out of the lab by busting through a few walls.
As Underdog walks through an alley trying to figure out what just happened, he runs into a bully dog Riff Raff and 2 other dogs. They tell Underdog to get out of their alley; Underdog doesn’t want any trouble, so he takes off. Unfortunately, he runs out into the street in front of the car. Dan, who’s driving the car, slams on the brakes and gets out looking for the dog. Dan realizes he didn’t hit the dog after he finds him hiding underneath the car. Dan decides to call the dog Shoeshine (I forget why) and takes him home for the night; as they drive off, we see a large dent on the car’s front bumper that only a dog could’ve made.
The next morning, Jack Unger (Alex Neuberger) makes up a note from his dad excusing him from school. Dan walks in and shows Shoeshine (fyi, I’m going to call Underdog Shoeshine when he’s not fighting crime) to Jack; Jack doesn’t really like Shoeshine at first. Later, Jack gets at trouble at school; apparently, the sickness he put on his note was “Monkey pox”. When Jack comes home, he sees that Shoeshine has totally trashed the place. As Jack scolds Shoeshine, Shoeshine tells him it was an accident. Jack freaks out since the dog is talking, but Shoeshine calms him down and tells him what happened. As Jack is still trying to figure everything out, they go to a park. Also at the park is the writer for the school newspaper, Molly (Taylor Momsen). Shoeshine immediately falls for Molly’s dog, Polly Purebred (voiced by Amy Adams). After they talk, Jack gets a Frisbee out of the trash and throws it across the field; Shoeshine runs at lightning speed to catch it. Jack is amazed by this and decided to experiment with Shoeshine; in a funny scene, he tries playing fetch with Shoeshine, but Shoeshine isn’t amused. Suddenly, Shoeshine gets some sort of “spidey-sense” and realizes Molly and Polly are in trouble. Shoeshine starts running (which turns into flying) and finds Molly’s purse is being stolen by two guys. Shoeshine makes it in time and saves the girl and her dog.
Jack completely forgets that his house is still a mess, so he and Shoeshine run back to the house to clean it up (yea, Shoeshine helps clean). As they clean, Cad stops by asking if they’ve seen his lost beagle; Cad gets suspicious, but Jack says he hasn’t seen him and closes the door. Unfortunately, Dan gets home and finds a broken window, shattered dishes, etc. Dan decided to take the dog to the pound, but Jack convinces him to let Shoeshine stay now that he likes the dog. Jack takes Shoeshine to the park again and decides to teach him some obedience tricks (sit, roll over, speak, etc.). As Jack and Shoeshine watch a baseball game, Jack comments on how his team are always the “underdogs”, and explains to Shoeshine that an underdog is someone who nobody expects to be great but is (Shoeshine can relate). Jack also gives Shoeshine a collar with his name on it, which makes Shoeshine happy now that he finally has a home.
Back at the lab, Cad helps Simon emerge from the sewer he’s been living in; Simon now has a crippled leg and scars around his brain. Simon is furious that his lab is destroyed and that Cad is too stupid to not find the dog, but Simon still attempts to rebuild his lab.
At the house, Shoeshine tries hypnotizing Jack to give him some food, but it doesn’t work. Jack sees a jewelry heist on TV, and is certain that Shoeshine can stop the heist. Shoeshine doesn’t want to do it, but Jack convinces him to do it. Shoeshine flies out of the house (into a fish kite) and towards the jewelry store. When he gets there, we see that Cad and a few henchmen are robbing the store (for Simon, obviously); Shoeshine (still wearing the fish kite) roughs up the henchmen, chases Cad away, and gets worried when he sees a fox skin scarf lying on the store’s floor (he thinks it’s alive).
The next day, Jack tells Shoeshine that the jewelry store incident was incredible and that he should become a regular superhero. Shoeshine doesn’t want to do it though because he’s afraid he’ll be noticed and that Simon will take him back to the lab. Jack doesn’t want Shoeshine to quit, so he goes into a story about how his dad used to be the greatest cop in Capitol City then quit for no reason. Dan was supposed to spend a lot more time with Jack after his mom died; however, Dan hasn’t really done a good job with that. To help with Shoeshine’s problem, Jack decides that Shoeshine needs an alter-ego to protect his identity. They try out a few costumes, but finally settle on Dan’s old college shirt (a red shirt with a white U on it) and a blue cape. Along with his alter-ego, they decide if Shoeshine needs to leave to help someone, he barks three times. They also settle on his name: Underdog.
For the next few minutes, a montage showing Underdog stopping different crooks cues up. As he takes down crime, he works on a few different catchphrases, but finally settles on the famous, “There’s no need to fear; Underdog is here.” Now would also be a good time to point out that whenever Shoeshine is Underdog, he speaks majority of the time in rhymes (just like in the cartoon). In the same montage, Molly looks a lot of research trying to figure out exactly who Underdog is.
One day, Jack takes Shoeshine to the library so they can meet up with Molly and Polly; Shoeshine calls it a “double-date”, which Jack immediately says it’s not. They have to work on Shoeshine’s secret identity, so he has to wear one of those dog cones around his neck. At the library, Riff Raff comes around and makes fun of Shoeshine’s cone. Shoeshine talks with Polly, who really just talks about how amazing Underdog is. Shoeshine tells Polly that Underdog will answer any call for help; that night, Polly stands out on her porch saying, “Help.” Underdog comes, and when he realizes there’s really no need to be there, he asks Polly out on a date. They fly over all over Capitol City and stop at an Italian restaurant to steal a couple’s “doggy bag” (yea it got a chuckle out of me). As they eat on top of City Hall, Underdog drops a meatball on a cat.
Meanwhile, Simon is getting furious that Underdog is getting recognized for the powers he gave him (and irritated since Cad is getting dumber and dumber). Simon comes up with an idea to lure Underdog out: Cad dresses up like an old lady and hangs out of a window of a tall building. It lures Underdog out, who flies in too fast and knocks Cad down. Underdog still saves him and brings him down to the ground; Cad takes off his old lady wig and puts a choke collar on Underdog. Underdog decides to take Cad for a “walk” he runs around the city at lightning speed, dragging Cad around. Cad goes back to Simon, who’s furious for another failure; however, Cad was able to rip Underdog’s collar off, revealing his address.
Later, Jack and Shoeshine head out to go meet with Molly and Polly. Dan asks if he can come, but Jack says he and Shoeshine will be fine. It’s obvious Dan is trying to spend time with Jack, but Jack just keeps blowing him off. Shoeshine calls Jack out on this, so Jack wants to apologize. However, Simon and Cad kidnap Dan and bring him to the lab. Simon now knows that Shoeshine is Underdog, so he sicks his 3 test dogs (all German Shepherds) on Dan until he calls to Underdog for help. Shoeshine hears Dan’s call and barks three times. Jack and Shoeshine both awkwardly run away from talking with Molly and Polly to go save Dan. They get to the lab, but they end up getting captured. Simon will let Jack and Dan go if Underdog agrees to give up his powers. Underdog agrees, and his powers are sucked out and put into blue pills. Simon forces to take Underdog to take one more pill, which returns him to his normal dog self.
Simon uses his new powers to transform his test dogs into super dogs. Simon goes to City Hall and holds the mayor hostage; his demands are a billion dollars, immunity from the law, and his lab restored. As the SWAT team arrives and people gather to watch, Molly and Polly sneak in after they see Cad enter. Back at the lab, Jack and Dan start arguing about Dan quitting the force. Dan finally admits he only quit because he was afraid of dying and leaving Jack alone as they escape the lab with Shoeshine.
Jack, Dan, Shoeshine get to City Hall and learn what’s happening. Shoeshine, who looks helpless now, suddenly smells a bomb. Shoeshine is skeptical of his smell, but ends up going inside via a vent. Shoeshine sneaks through the vent until he falls through and lands on a globe. The globe goes out of control, knocking the mayor unconscious and knocking a few of the blue power pills out of Simon’s pocket. Shoeshine eats one and becomes super dog again. As the test dogs chase Underdog around, Molly finds Cad strapping a bomb to the top of the building. Polly barks at Cad, which causes Cad to find both of them and tie them up where the bomb is. Back inside, Simon takes a pill himself and now has all the powers Underdog has. He gets the upper hand by throwing a Frisbee-looking shield at Underdog. Simon is now ready to detonate a bomb, which will release a toxin in the air forcing all citizens to obey him. When the test dogs are set to finish Underdog off, he talks to them and asks if they really like working for Simon. Simon yells at the dogs, but they get annoyed when he tries to interrupt their talk. Finally Underdog convinces them to hold Simon down as he goes outside to get his outfit. As the mayor gains the strength to leave City Hall, Dan decides to go inside and capture Simon.
With two minutes left on the bomb, Underdog heads up to save Molly and Polly. Underdog takes the bomb and digs a DEEP hole to put the bomb in (like, “center of the earth” deep). The bomb goes off, sending Underdog into space (where an astronaut sees him). Underdog falls back down to Earth, causing a crater. For a minute, we think he’s dead……..but just like any Disney movie, he’s not. The town celebrates.
A few days later, Simon is shown to his lifetime cellmate; he screams in agony when he sees it’s Cad. Meanwhile, Jack and Molly are the park (yet again). Polly still cannot stop talking about Underdog, which bums Shoeshine out. Riff Raff comes over to bully Shoeshine again, but Shoeshine gives a mega bark that shaves all of Riff Raff’s hair.
The movie ends with Shoeshine jumping up in the sky and flying around as Underdog.